1. Field of the Invention
The present invention relates to illumination optical systems, in particular to illumination optical systems used as illumination units or the like for image-taking apparatuses.
2. Description of Related Art
Conventionally, illumination optical systems condense a luminous flux emitted from a light source within the necessary irradiation angle range with high efficiency, using an optical member such as a Fresnel lens or a reflector. The shape of the above-described optical member is optimized, and the shape of each portion of the illumination optical system is defined so as to provide predetermined light distribution characteristics.
On the other hand, as digital cameras, portable devices and the like equipped with such illumination optical systems have been increasingly downsized in recent years, miniaturization of the illumination optical systems has also been advanced. Then, there have been various proposals for suppressing the optical performance deterioration of the illumination optical systems resulting from miniaturization.
For example, a compact, highly efficient illumination optical system has been proposed which utilizes the total reflection of a prism disposed in the vicinity of the light source, as disclosed in Japanese Patent Application Laid-Open No. H10 (1998)-115852 and Japanese Patent Application Laid-Open No. 2000-250102.
On the other hand, there has been a proposal aimed at improving the characteristics of the optical material of the illumination optical system by dispersing elementary particles in the optical member. For example, Japanese Patent Application Laid-Open No. H11 (1999)-345512 has proposed a surface light source device in which layers with different refractive indices and nonuniform thickness are formed by dispersing fine particles with different refractive indices in the base material of the optical member, so that the emitted light becomes uniform.
In addition, Japanese Patent Application Laid-Open No. H8 (1996)-201807 has proposed an illumination optical system for liquid crystal display back lights which intentionally diffuses a luminous flux by uniformly dispersing, in the base material of the optical member, particles with different refractive indices which are not compatible with each other, thereby guiding the luminous flux to a desired direction with high efficiency.
Furthermore, Japanese Patent Application Laid-Open No. H11 (1999)-188268 has proposed a technique for providing a refractive index distribution by including a metal oxide inside a glass tube and gradually changing the content of the metal oxide inside the glass tube from the inner surface side.
As mentioned above, electronic devices such as digital cameras are increasingly downsized and the illumination optical systems incorporated in such devices must be also miniaturized. However, even with the use of the above-described technique, it is difficult to achieve further miniaturization of the illumination optical system, while maintaining their existing optical characteristics.
More specifically, PMMA (polymethyl methacrylate), which has excellent optical characteristics and sufficient mechanical strength as an exterior member, is widely used at present as the material of an optical member constituting an illumination optical system. The refractive indices of major optical resin materials, including PMMA, used as optical members are approximately 1.5.
However, in order to achieve miniaturization of the illumination optical system, it is essential to use a material having a higher refractive index than ordinary resin materials.
On the other hand, in the optical system as disclosed in Japanese Patent Application Laid-Open No. H10 (1998)-115852 or Japanese Patent Application Laid-Open No. 2000-250102, the distance between the light source and the optical member is extremely short. This presents the possibility that some optical materials may be deformed or discolored by heat generated by the light source and thus may be unable to maintain the shape, optical characteristics and the like of the optical member. In view of such situation, it is necessary to take into consideration not only a high refractive index, but also heat generated by the light source, when selecting the optical material.
Further, in each of the optical systems proposed in Japanese Patent Application Laid-Open No. H11 (1999)-345512 and Japanese Patent Application Laid-Open No. H8 (1996)-201807, relatively large, transparent elementary particles having a thickness of several micrometers and a refractive power different from that of the base material of the optical member are dispersed. However, this is basically aimed at uniformly diffusing light, and cannot simultaneously improve the optical characteristics, mechanical characteristics and the like of the optical member.
Additionally, the light source proposed in Japanese Patent Application Laid-Open No. H11 (1999)-188268 is extremely expensive as it uses glass as the base material, and a refractive index gradient is provided for the glass tube, which serves as the light source. Therefore, the proposal does not relate to the optical member which exerts an optical action on the luminous flux emitted from the light source.